The present invention relates to microwave probes for tissue ablation and in particular to a microwave antenna providing improved localization of tissue heating.
Microwave and radio frequency ablation may be used to treat tumors, for example in the liver, in patients who are not eligible for surgical removal of the tumor. In microwave ablation, electrical energy with a frequency in the megahertz to gigahertz range is directed into the tumor using a specially designed antenna (ablation probe). The microwave energy received by the tumor and surrounding tissue is converted to heat which destroys tumor cells. Microwave ablation does not require a separate ground pad attached to the patient, and thus may be distinguished from ablation at lower frequencies often termed radiofrequency ablation.
The heat energy deposited into the tissue for a given microwave power and duration may be characterized by the Specific Absorption Rate (SAR) of tissue in the vicinity of the probe. The SAR pattern for a microwave probe may therefore be used to characterize a size and shape of the ablation region. In many applications, the ideal SAR pattern of the microwave probe will be concentrated at the tip of the probe (the portion located in the tumor) and not along the shaft of the probe such as may affect healthy tissue or preclude the use of thermal ablation as a treatment option. Such problems may be reduced, but not eliminated by cooling or insulating the shaft of the probe to decrease thermal conduction between the tissue and the shaft, the latter which may be heated by resistive losses in the transmission of microwave energy. Such thermal conduction provides tissue heating in addition to that produced by radiated microwave energy.
A “dual slot antenna” described in C. Brace, Dual-Slot Antennas for Microwave Tissue Heating Parametric Design Analysis and Experimentation Validation, Med. Phys. 38(7) 4232-4240 (2011), provides an experimental design for a microwave probe providing a distally concentrated SAR at the boundary of the ablation region. Modifying this design by providing thermal cooling of the probe shaft and robust high temperature insulating materials, such as a ceramic antenna support structure, significantly degrades this desirable SAR pattern.